NICKEL-CATALYZED [2-PI+2-PI+2-PI] (HOMO-DIELS-ALDER) AND [2-PI+2-PI] CYCLOADDITIONS OF BICYCLO[2.2.1]HEPTA-2,5-DIENES

Active catalysts which promote the homo-Diels-Alder (HDA) cycloaddition with a variety of electron-deficient olefins with bicyclo[2.2.1]hepta-2,5-diene (norbornadiene, NBD) have been developed. The nickel complex and the additives (e.g., Ligands, reducing agents) influence the activity of the catalyst. Dienophiles which participate in this cycloaddition include acyclic and cyclic enones, lactones, sulfones, and sulfoxides. The dienophile substituent, the catalyst, and the temperature affected the exo/endo selectivity in the HDA reaction. A diastereoselective reaction with an optically enriched vinyl sulfoxide led to the synthesis of an optically active deltacyclane. The regio- and stereoselectivity of the HDA reaction between 2-substituted norbornadienes and electron-deficient dienophiles has also been studied. The substituents on the diene and dienophile as well as the ligands were found to exert a dramatic effect on the selectivity. With very reactive dienophiles, an alternative [2 pi+2 pi] cycloaddition was discovered for 2-substituted norbornadienes. In some cases, the [2 pi+2 pi] cycloaddition can occur with high chemo- and regioselectivity and moderate levels of stereoselectivity.